Zebra finches are socially monogamous birds. This means that, like humans, they generally form lifetime relationships with a single partner. Also like many humans, they’re known to sleep around.

For males, this makes sense, from an evolutionary standpoint. The males can increase their fitness by passing on more of their genes to the next generation if they father offspring with many females. Females, on the other hand, don’t seem to benefit. Zebra finch males will put less effort into helping raise the youngsters if they find out that one of them is the milkman’s child. And females have a higher risk of sexually transmitted diseases which can negatively impact their fertility. So why do they step out at all? After observing over 1,500 zebra finches, scientists think they have the answer: they inherit the cheating gene from Dad.

Their aim was to see which hypothesis best explained why these females were actively looking for males besides their partner.

Is it because there is only one set of genes for being “in the mood”? In that case, a female who righteously rejects advances from other males might also reject the advances of her partner, making her effectively sterile. The price of a happy marriage would then be a little bit of infidelity.

Or, could female promiscuity be explained by a correlation between males and females? If the same genes are responsible for making unfaithful males and females, and if the benefit to males outweighs the risks to females, then these genes would spread in the population.

The researchers found evidence supporting the second hypothesis, but not the first one. They took videos of individual birds’ mating habits and kept track of how often they mated with their partner or with another bird. They also did genetic tests to determine the paternity of the chicks from females who were promiscuous. What they found: there was no correlation between how willing a female was to mate with her own partner vs. another male. But, there was a large correlation (about 60%) between female cheating and male cheating.

This couldn’t be explained by females learning from their parents. The chicks that were followed in the study were raised by foster parents. The researchers switched them as eggs to control for effect of family environment. Even so, female and male extra-pair mating behavior (or cheating) was genetically linked.

That means those sneaky females probably inherited their habits (mostly) from Dad. Males who carry the “cheating” gene (or set of genes) will pass it on to their sons and daughters. Sons get an obvious benefit: they sire more offspring. Daughters may get some benefit (genetic variability in offspring), but these do not appear to outweigh the costs for zebra finch females. Thanks, Dad!

Isn’t evolution wacky?

(On the flip side, males who don’t cheat won’t pass on this gene. So, faithful females likely have Dad to thank as well.)

In what ways have you been influenced by your parents, either genetically or otherwise?